1. Field of the Invention
The present invention relates to a thick-film paste used in the preparation of a ceramic circuit substrate having on its surface an external resistor covered with a glass overcoat and to a ceramic circuit substrate in which the paste is used. More particularly, the present invention is concerned with a ceramic circuit substrate having an external resistor which stably maintains an accurate resistance value obtained by trimming.
2. Description of Prior Art
Besides the internal resistor disposed between the layers of a multilayer circuit substrate, a ceramic circuit substrate for use in integral circuits is provided with a circuit comprising a conductor pattern and an external resistor printed on the surface of the ceramic circuit substrate, which contributes toward imparting an advanced function to the ceramic circuit substrate and reducing the production cost.
In the formation of a thick-film resistor on a substrate surface, generally, a conductive substance is added to a glass composition, rendered pasty, printed and sintered into the desired resistor. In the formation of the resistor, occasionally, printing is effected so as to cover the resistor with a glass material and followed by firing to form an overcoat in order to protect the resistor and to improve the weather resistance thereof. The obtained resistor has its resistance value finely adjusted by laser trimming, etc.
One of the important properties of the thick-film resistor is high voltage pulse properties [ESD (electrostatic discharge) characteristics]. The conductance of the thick-film resistor relies on a thin glass layer formed between the conductive substances in glass. Application of a high voltage to the thick-film resistor destroys minute conductive paths, thereby causing resistance value changes. One possible method for improvement comprises refining the particle size of the glass powder to thereby improve the dispersion thereof in conductive particles with the result that the number of conductive paths formed out of the glass and conductive particles (e.g., RuO.sub.2 particles in a RuO.sub.2 -based resistor) is increased to thereby decrease the amount of electric charges which flow through one conductive path, so that destruction of the conductive paths would be avoided and the resistance value changes minimized. However, with respect to the resistor co-fired with the glass overcoat, excess reduction of the particle size of the glass in the resistor tends to hamper the decomposition of a binder used in a paste, so that its decomposition cannot be completed before the sintering of the glass overcoat and hence the binder is confined by the glass overcoat to remain in the resistor as carbon, which is oxidized into CO.sub.2, etc., and expands with the temperature rise to thereby form bubbles in the resistor.
Generally, a resistor used in a ceramic circuit substrate is formed by firing a resistor at 800 to 900.degree. C., printing a low-melting point glass overcoat thereon, and firing at 500 to 600.degree. C. In accordance with the miniaturization of electronic appliances and the higher-density packaging therein, there is the tendency that the ceramic substrate is also provided in multilayer form to comply with higher-density packaging and that use is made of substrate materials which each have a low coefficient of thermal expansion for mounting silicon chips thereon. As such circuit substrates, low-temperature firable substrates are used,
Most of the low-temperature firable substrates contain Ag and Cu in the inner layers, so that, for decreasing the frequency of thermal expansion and shrinkage thereof to thereby give a circuit substrate of high reliability, the number of firing steps should be minimized. Further, for conformity with the thermal expansion of the circuit substrate, a glass composition (including, besides a glass powder, an additive powder, such as alumina powder) of a low coefficient of thermal expansion should be used in the glass overcoat as well, However, the low-melting point glass has a drawback in weather resistance, so that it is required to use a glass having a melting point as high as about the temperature employed for firing the resistor.
It is therefore apparent that a resistor capable of being co-fired with the glass overcoat is desirably employed in a ceramic circuit substrate of a low thermal expansion coefficient having a multilayer structure. However, the co-firing of the resistor and the glass overcoat brings about the tendency that the glass overcoat confines the bubbles generated from the resistor, thereby causing the bubbles to remain within the sintered resistor. When the bubbles remain as confined in the resistor, a problem occurs such that a very close access of a trimming edge to the bubble at the process of laser trimming produces cracks therebetween, resulting in the formation of a resistor lacking in stability in resistance values.
The above situation will be described with reference to the drawings. FIG. 1 is a plan view of one form of a conventional external resistor disposed on a ceramic circuit substrate, and FIG. 2 is a sectional view thereof. A wiring material, such as a metal paste, is printed on a ceramic substrate surface 1, thereby forming a conductor pattern 2 on the surface. Part thereof constitutes electrodes for a resistor 3. The resistor 3 is composed of glass components having a conductive material such as a metal added thereto. The upper part thereof is covered with an overcoat 4 composed of glass materials which may include an alumina powder or the like in addition to a glass powder. The resistor 3 and the overcoat 4 constitute an external resistor 7. The overcoat 4 may either cover each individual resistor 3 a little wider than the same or uniformly cover a wide area of not only a plurality of resistors 3 but also a conductor pattern 2. When the overcoat covers such a wide area, via holes can be provided at suitable positions to thereby attain continuity with the outside.
The co-firing of the overcoat 4 and the resistor 3 prevents the bubbles 6 generated in the resistor from escaping outside because of the presence of the overcoat 4, thus causing them to remain confined in the resistor 3. Laser trimming of such an external resistor 7 leads to the formation of a trimming channel 5 in the overcoat 4 and the resistor 3, as shown in the figures.
Although laser trimming is generally conducted while measuring the resistance value exhibited by the resistor, the presence of the bubbles 6 not only interferes with such precision trimming but also generates microcracks upon access of the trimming channel tip to the bubbles 6. Also, even if there is no occurrence of cracks during trimming, cracks may occur because of the bubbles during its use as a product. Thus, the presence of bubbles in the resistor renders the resistance value exhibited by the resistor inaccurate and renders its resistance value after adjustment unstable.